Transmissions with several gears have at least one drive shaft, which in the case of a motor vehicle is usually driven by an internal combustion engine connected to the transmission drive shaft via a shiftable clutch often configured as a dry friction clutch. The transmission output shaft is for example connected via a differential gear to the drivable wheels of the vehicle. As internal combustion engines can only be operated within a relatively low speed range, the transmission is used for adjusting the speed of the drive shaft at a given speed of the vehicle and/or at a given speed of the output shaft such that the internal combustion engine may work at the most favorable operating level.
Besides certain special forms, like e.g. continuously variable transmissions, a customary transmission for passenger cars has at least two, but mostly 4 to 7 fixed transmission ratios or gears, apart from the drive shaft and output shaft, a reverse gear with reversal of rotation direction being nearly always provided. Transmissions with even considerably more gears are used in freight vehicles.
The individual gears are either shifted when certain gear pairs (or also equivalent transmission means like belt sprockets) are shifted into the flow of torque, while other gear pairs do not actively rotate with the transmission of torque. For this purpose, certain gear wheels may be arranged displaceable but rotationally fixed on shafts and be displaced when the allocated gear is engaged such that they mesh with another gear wheel. Alternatively, the gear wheels may permanently intermesh with their allocated gear wheels, and the torque transmission between the gear wheel and the shaft supporting the gear wheel, e.g. via a shiftable claw clutch, may be shifted between a freewheel and a rotationally fixed transmission.
In both cases, it is possible that, in particular with stationary vehicles and consequently motionless output shafts, either the end faces of two teeth of gear wheels to be engaged with one another, or of claws of a claw clutch to be closed, abut one another with their elevated parts and the requested gear cannot be engaged.
In order to solve this so called tooth abutment condition, beveling the end faces of the gear wheels and/or claws to attain a radial, torsion force component from the active axial force, and at least rotating a gear wheel and/or claw further so that the gear wheels or claws neatly mesh is known. This is, however, associated with considerable production expenditure.
Furthermore, adjusting the synchronization of the involved gear wheels or claw clutches by means of different devices and methods such that meshing is aided by a rotational speed difference is known. Admittedly, the efficiency of this approach without simultaneously beveling the face ends is limited just because when the end faces touch, their friction generates a braking torque that in turn favors a tooth abutment condition. Correct shifting may in fact be favored this way, but only restrictedly guaranteed, if the shifting speed will not be set undesirably low. Therefore, there have been different proposals on how a tooth abutment condition may be overcome as rapidly as possible.
For this purpose, relative rotation of the gear wheels or claw clutches involved is required. Apart from very complex solutions with internal drives, it is presently suggested to produce this relative rotation by rotating at least one of the drive shafts or output shafts, while possibly simultaneously overcoming or at least reducing contact pressure of the gear wheels or claw clutches involved. As this has to function reliably with a standing vehicle and consequently standing transmission output shaft, and decelerating the vehicle for overcoming the tooth abutment condition is not desirable, the clutch is preferably engaged for this purpose such that torsion of the transmission drive shaft results and shifting is again attempted at the same time or immediately afterward.
The problem with this is that the torque to be transferred by the clutch for the desired positive or negative acceleration of the drive shaft depends on a large number of parameters. Among others, those parameters are the vehicle speed and the speed of the transmission output shaft depending upon it, the rotational speed of the engine output shaft and the operating level of the engine that in turn depends on the type, age and primarily the temperature of the transmission oil.
Therefore, according to the current experience, in order to be able to overcome a toot abutment condition in the transmission under all possible or usual practical conditions, the clutch has to be engaged to an extent such that even under unfavorable conditions, which require a high torque at the drive shaft for overcoming the tooth abutment condition, this torque is at least achieved. This, however, entails that in the case of tooth abutment conditions that are less difficult to overcome a still considerable torque develops, and the motor vehicle is subject to an audible and even perceptible jerk that is unpleasant for the passengers and damaging to the claws or teeth colliding with each other.
From U.S. Pat. No. 6,769,523 B2 a method has been known where a controlling device detects a tooth abutment condition and consequently continuously varies the position of the clutch until the tooth abutment condition is overcome or an abort criterion is met. More precisely, different attempts are made with different clutch positions, where the position of the clutch, starting at relatively small excursions with intervals, is displaced with more strength to its engaged position. Consequently, easy to resolve tooth abutment conditions may be overcome with low torque being transferred by the clutch, and the above described undesired effects can be to a large extent avoided. In further developments, it is provided that certain parameters, like the transmission oil temperature, are recorded and evaluated to be able to possibly adjust the most suitable clutch position for overcoming the tooth abutment condition more rapidly. Although this method represents a definite improvement compared to the previously described method, this known technical solution is not entirely satisfactory.
This is probably related to the fact that the clutch has been designed for the transmission of the maximum output of the drive engine, which makes exact positioning of the clutch for the transmission of the desired, comparatively very low amount of torque for overcoming a tooth abutment condition very difficult. In addition, the clutch is subject to considerable wear during its life time, which has to be taken into account in the same way as the rapidly changing parameters, like the operating temperature of the clutch, which may cause significant changes in the clutch behavior because of the thermal expansion in the adjustment range of interest here.
Further, the relation of the contact pressure of the friction elements to the torque to be transferred thereby may be modified by changes in the temperature of the friction surface, but also, for example, by overheating-induced vitrification, contamination of the friction surfaces or other circumstances. After all, the adjustment mechanisms of the clutch are necessarily likewise designed for the adjustment path required for the entire driving operation, whereby either a complex and thus expensive as well as potentially error-prone regulation of the clutch position is required for accurate adjustment in the currently relevant range of a relatively low torque transfer, or the positioning accuracy is quite low for the purpose described herein.
Moreover, when a tooth abutment condition is detected, the clutch has to be adjusted to a desired value in the first instance, and the engine speed as well as the torque adjusted at the same time, and/or taken into account when determining the target position of the clutch. The time span required for this purpose is short, but is still perceived as an unpleasant period by the passengers because of the repeated attempts that may be necessary to overcome the tooth abutment condition.